Morrison
Formation

Fossils
of the Morrison Formation, Western United States and Canada

The Morrison Formation is a Late Jurassic
sedimentary rock sequence in the western United States and
Canada, from which numerous dinosaur fossils have been discovered.
The
Morrison Formation comprises mudstone, sandstone, siltstone
and limestone and is light grey, greenish gray, or red. Most
of the fossils occur in the green siltstone beds and lower
sandstones,
formed by the rivers and floodplains of the Jurassic period.
While centered in Wyoming and Colorado, trhe formation also
has outcrops in Montana, Saskatchewan, Alberta, North Dakota,
South Dakota, Nebraska, Kansas, the Oklahoma and Texas panhandles,
New Mexico, Arizona, Utah, and Idaho. In total, it covers some
600,000 square miles, although but a small portion is actually
exposed.

The
formation was named for Morrison, Colorado where its first fossils
were discovered by Arthur Lakes in 1877. That same year, it
became the center of the Bone Wars, a fossil collecting rivalry
between paleontologists Othniel Charles Marsh and Edward Drinker
Cope. Radiometric places the age at between between 148 and
155 million years (i.e., the Oxfordian, Kimmeridgian, and earliest
Tithonian stages of the late Jurassic). It is thus similar in
age to the Solnhofen Limestone of Germany.

Fossils
from the formation provide the ability to describe an environment
that was dry and lacking of grasses, flowering plants, and trees.
Rather, conifers were the dominant flora along with ginkgo,
cycads,
tree ferns, and horsetail rushes. Much of the fossilized vegetation
was riparian, living along the river valleys. Insects were very
similar to modern species. Fossils of fish, frogs, crocodiles,
salamanders, lizards, crayfish, turtles, pterosaurs, clams are
also found. Hundreds of dinosaur fossils have been discovered,
including Camptosaurus, Ornitholestes, Stegosaurus, Hesperosaurus,
early ankylosaurs, and most importantly, a diversity of sauropods.

Sauropods
that have been discovered include the Diplodocus
(most famously, the first nearly-complete specimen of Diplodocus
carnegiei, that is now exhibited at the Carnegie Museum of Natural
History, Camarasaurus (the most commonly found sauropod), Brachiosaurus,
Apatosaurus (also wrongly known as Brontosaurus), Barosaurus,
the uncommon Haplocanthosaurus and the Seismosaurus. The very
diversity of the sauropods has raised some questions about how
they could all co-exist. While their body shapes are very similar
(long neck, long tail, huge elephant-like body), they are assumed
to have had very different feeding strategies, in order for
all to have existed same time and place. About 75% of all Allosaurus
fossils have been recovered from the Morrison Formation totalling
more than sixty partial and nearly-complete specimens, including
the first holotype specimen.